fdt: Expose the model, compatible and freebsd dts brandind as sysctl

This make it easier for script to get the hardware on which they are running.

MFC after: 1 month
Sponsored by: Diablotin Systems
Differential Revision: https://reviews.freebsd.org/D31205
Reviewed by: imp
Should be ok on powerpc: jhibbits (over irc)